Radio altimeter system



Jan. 26, 1965 R. s. NoRRls, .JR

RADIO ALTIMETER SYSTEM 2 Sheets-Sheet 1 Filed Aug. 26. 1960 Jan. 26, 1965 R. s. NoRRls, JR 3,167,766

RADIO ALTIMETER SYSTEM Filed Aug. 26, 1960 2 Sheets-Sheet 2 Tlc'l.-

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DH/i United States, Patent O M 3,l67,766 RADI AL'HlfnETEi-l SYSTEM Russell Snyder Norris, Sir., State College, Pa., assigner to HRB-Singer, inc., State Coliege, Ia., a corporation of Pennsylvania Filed Aug. 26, 1960, Ser. No. 52,279 8 Claims. (Ci. 343-14) This invention relates to radio altimeter systems and more particularly to frequency modulatedl continuousV wave radio altimeters for use in blind-landing systems.-V

A radio altimeter is basically a distance measuring system in which the transmitted energy is reflected by the terrain over which the mobile unit is disposed; the distance to the ground from the mobile unit being deter@ mined from the information contained in the received echo signal. In the past, one common radio altimeter system frequency modulated the carrier signal emitted by the transmitter. Av receiver, mounted in the mobileV unit, detected the FM echo signal return .and compared it with a low amplitude sample signal derived over a direct path from the transmitter. Due to the longer path of the echo signal, a finite time delay occurs between the received echo signal and the direct path signal. Since the frequency of the transmitted waves are periodically varied in a known manner, the difference I-F frequency of the signals received over both paths is proportional to the rate of transmitter frequency change and to the path distance of the echo signal andthus may be'used as an indication of the height of the mobile unit abovel the ground.

It has long been recognized that such a radio altimeter system inherentiy contains errors as a result of the method employed in varying the transmitter frequency, and also l mits ground detection and -demodulation of the transi mitted signal to obtain an indication of the aircraft' as a result of the quantizing characteristic of the use of an averaging frequency counter to determine the difference frequency; Theseerrors result in lambiguities in the indicated altitudes for discrete increments of altitude. Theoretically such errors may be reduced to negligible proportions by increasing the limits of the carrier frequency deviation. However, from a practical point of View, such an arrangement is prohibitively wasteful of available frequency spectrum.

More recently a radio altimeter system, designed primarily as a terrain clarance indicator, was developed in which :the carrier wave was frequencymodulated in a nearly symmetrical manner. In that system, the peak-topeak deviation of the carrier wave varies inversely with altitude, when the aircraft was above 200 feet, to maintain a nearly constant frequency difference' between the received echo signal and the direct path signal. This known system provides the narrow band operation necessary for operations at high altitudes, but at altitudes below 200 feet the system contains a fixed error of about three and oneV half feet. Above 200 feet the *fixed error varies lwith altitude due to the change in frequency deviation with altitude.r As a terrain indicator the system is quite satisfactory kbut the fixed error at low altitudes severely limits its usefulness as an altimeter for use in blind-landing systems. .l

In order to achieve the precision required Vin aircraft blind-landing. systems, itis important that the altimeter fixed errorsbe eliminated or be reduced to negligible proportions. The requirements for a low altitudeand blind landing flareout altimeter system have been stated vby the 3,167,766 Patented Jan. 26, 1965 ICC 2 systems are unsatisfactory for use in blind-landing systems, and for this reason the present blind-landing systems generally utilize visual contact with the ground'just prior to touchdown.

.One of the objects of this invention, therefore, is to protion of the height of the aircraft. v

One of the features of this invention is theprovision of an FM-CW radio altimeter system for use in blindlanding systems, in which two overlapping feedback modes of operation are incorporated, both automatically con.

trolled, to eliminate fixed or step errors at low altitudes. The feedback path in each mode of operation is used automatcally to control the modulationfrequency.,such that the modulation frequency 0f the transmitted lsignal is iii-- versely yproportional tothe aircraft altitude. This per- FIG. 2 is a schematic diagram of a novel microwave 1 discriminator for use in the radio altimeter of this nal produced by crystal mixer 8 is coupled to a band-pass` the transmitter 1 is coupled through a directional coupler 3 to a transmitting antenna 4. The directional coupler 3 permits the majority of the radio frequency power. developed' by transmitter 1 to be deliveredv to the transmitting antenna-4.

The modulated transmitter reflected. VA portion of the ground reflected signal visV incident on the receiving antenna 5. The physical mounting` ofthe transmitting and receiving'antennas 4,-5 is 'ar-v ranged to enable a controlled'level ofthe transmitted signal to be coupledrdirectly tothe receiving antennaV 5, The receivingl antenna 5 is coupled` to a. hybrid-'ring 6,

having a termination 6a in one branch -The ring 6 serves l to divide the received power equally "into" two isolated'llg channels. The signal provided by `each channelfof the hybrid 6 consists of the ground-return and direct-leakage signals.

One output from hybrid-'ring A6 is'coupled throughV aVr crystal diode microwave switch 7 to -a` crystal mixer 18."

The ground-return signal and the controlled leakage signal are operated on bythe non-linearpro'pertiesV of the crystal rniXer 8 to produce a signal representing the difference frequency between the controlled leakage signal and ground return signal. f This Ydifference frequency signal is dependent` on the original modulating frequency,` the frequency deviation produced by the modulating signal and 'I I the delay time of the ground return signal with respect to the transmitting signal. The difference frequency sigsignal is radiated by the transmittingantenna 4 toward the ground where itis .quencya Y.

' In addition,

i factorily operate from less .than `50 to more than 10Q() foot altitudes.- Moreover, itis apparent'that eitherV theVV i *system detection'threshold. y. Y

Afg'ainreferring Vto FIG.Vv 1,5 thels'econdY output terminal; of hybrid ring 6 iscoupledto one inputfof asecond' hybrid Y ringV 14. A kksample of` the transmitter signal derived from directional coupleur?, is :coupled VthrouglranTadjustableV Y Y attenuator 1S [and 'aiprecisionlength ofntrangsrni'ssion' line,k 16 to therse'conrd input ter-minalof hybrid ring 14.Y Inlthe tudeof the directleakagesign'al; n -v y Y Hyhridjring'jcouplers provide'a resulting .voltage level at'one outputterrninal representing the sum ofthe 'two fx-finput signals;` fandf'alvoltage--level at the'other outputter- VY'n'ii'nal, representing 'the diierence between'theftwoinput signals if'a Vzeroy or lSO-degr'ee; phasedilerence exists between the -two signal sources, havingfthe same frequency, applied to thet'two, input terminals. Thisis accomplished by ythe vprecision transmissiorrrlinef V155 whichY V"delays the' A Y f phase lof the transmitter' signal sample isuiicientlyy to f achieve the desired 178():degreephaserrelationwith'respect;Y 1 toithefdirect.leakage' signal'.-v V',-lh'usgat one'utputterminal V of"hybrid,ringf114, the-.dire Y cancelled, lwhile at "the othe loutputterniin'alithe sumofY .the 'flirit Y f VIn theY presence offal reflecting groundtherefore onlyj t e ground-return'signalappelarsat one` output terminal Y Y .of'th'ejhybrid ringY rvThis output termi-nal vis Vcoupled lttrol loop hasV noinhe'rent limitations,'such-asfiXedzerrorf` f" l i Y lter 9 to remove lowt'requencyl noise and high frequency j mixing products.Y Thefltered output from band-pass filter KY9 is coupled tolirniting amplifier 10 which serves toreduce Vamplitude', 'uctuations resulting fromy varying ground re'- flectivity andl other causes. rfrorri limiting'amplier 10 Vis then applied to a'frequency Y discriminator 11 which has agclenter frequency and an approximately logarithmiofrequency characteristic. The

The filtered, limited signalYV arcanes l l through a signal-levelrsampler 17 consisting of a direc- 'tional coupler .and video detector, toa crysta1-diode, volt,-Y

output froml discriminator ll'isiiltered bylow passvilter 12 and-'coupledtothebiascontrolcircuit 13. The output ofthe b iascontrolV circuit 13 l'comprises a'D.C,. control voltage which.` is applied .as the input to the voltage coni Y vtrollel variable frequencyoscillator 2 to-V varyitsiretheY biascontrolz circuit V13 provides 1an analog Vvoltage Voutput which ycanfbe utilized-Vl by.an1ancil lary computeror other equipment. Thus,`}this portion of v the radio al-timeter system of'this inventionrepresents a closedjloop arrangement for automatic `frequency control in which ,fthe aircraft altitude (distance of the ground returnsign'alfpath), modulation frequency and'diiierenceV frequency are ythe, variables.V j i VThe autoinatic frequency fcontrol loop iunctionsto Yrnaintairrga 'constant frequencyditerence (frequency tracker.) jfbe'tween the v,direct and ground. return signals Qfby'controlling the Afrequency of`the"modulatior1 energy'. n' fThef modulation frequency is inversely proportional toV Y altitude; Y'lfhe'automatic frequency control loop portion .i

ofthe radio altimeter Vsystem of Vthisinventioncan satisage-variable, microwave attenuator 18s, and then'toa microwave d iscrirninatonw.Y The opposite'loutput ter- Y minal ofhybrid ring 14,k at which the'sum of the trans rnitterV sample and direct' leakage signals appears', isfcou- Y pled to a video'detector 20. The detected signal output;VVV vfrom video detector 20is applied to van alarm Vsystem 20a which indicates failure of the transmitter.V

' The output ofthe signal-level sampler'17 also is coupled to a bias control circuit 21 and to athreshold circuit 22.

@At some altitudel between GandV 50 feet, the ground return signal amplitude is above the detection threshold of signal- VVlevel sampler 17k and'rnicrowave discrirninator 19 (c'omf i cuit 22, which in turn causes therdiode microwavejswitch f7' toy openfdeactivatingthe automatic frequency control loop. YThe detected output signal from the 'signal-,level rsaniplerli" .alsocontrols the bias voltage from the bias v control circuit 21,Yapplied to the voltage-variable micro,

` 1 waveattenuator 18, to provideV a degree of'lirniting of the ground return signal prior to FM demodulation bythe microwave discriminator. Limiting of the ground return 1^ i signal is desirable in orderV to minimize variations inthe s microwave discriminatoroutput due to VaryingV ground 1 f reflectivity.V c YReferring to EEG. 2 ofthe drawing the unique micro- 1 kwave discriminator 19 for use Yin the-radio altim'eterf ofv modulation frequency orthe'DC, control voltage can be y utilized to Vprovide, an indicationyof` altitude rather than Y t using thedifference frequency'.asinftheipriorart systems. j

-In'the second vroperating rno'dethe radio altimeterof i 'i this'i-nvention compares the-phase of the recovered imoduf f `*lation Waveformof thegroundlreturnfsignal against the" "i phase of the transmittermodulating waveform ina closed I1 Vloop which'. acts to maintain a constant phase difference between'thetwowaveforrns. The'use of :asinglereceivingfan'tenna for both modes of operation requiresffor absence ofiany Yground returnv .signal, attenuatorf15 is V:adjlklstedl to equate :the amplitude ofthe transmitter.; signal sample, derivedl yfrom directional. coupler leakage'insignal'. is jeiectively/ .leakage y andf transmitter sample signals. iappears.'

this inventionzis comprised 'of ahybrid ring 26, a detecf` fr tor 27,- and apredeterniined length of transmission line;V 2e. VThe frequency modulated microwave signal from the volta'ge-variable,microwaveattenuator l-isV coupled to the input/terminal of thehybrid ringfti., The input signal is split into two equal components, traveling in opposite `directions through ,the closed loop Vtransmission 'lin'e28; "r '40' ring may'includeiin one branch 29 the usual termination,

'In accordance with wen imbwn principles,";fhehybrid.;

and the transmission lineiZS mayalso includefthey usual l *the radio altimeter is entirely dependentlupon theautor v..atte'nuaftors Strand 3l( The detector 27: is disposed ad-e j jacent to one arm 32, of the hybrid ring andtherpredeter- 1 Vminecllength of transmission line 28 couples Vthe oppositev `end1 ofY the detector 27; to the Voppositehybrid'ring amr` f Y .33. modulating signal plus noise components.

The output of the detector 27 contains the original" Y VReturning now to FIG, l., the output of the'discrirninaAf-'r'f to; 19 is coupledto 'the AphaseV compensating videoarnplii Y i tier 243 and then toa phase comparator 24.- The refer; VV n Y kence signalinput'for the phase comparator 24is lobtainedJ from the .Qntputgjof thelvoltagefcontrolied, variable;'fre-r f quency oscillator, Zover-'Iine "2a." The 'outputfwofjheU phase comparatorll represents the diiierence infph'ase f between thetransmitter' modulation signal from oscillator VVV2 andthe ground;retu rn signal." This output isk coupled L toa 'lowpass lter 25 Vand then to the lwhichjhas been described; i .Y `i Thus, inithis vlast described mode of operation Vitfisjap-v V.liaren't that the closedloop represents an automatic phase Vcontrol Aservo in contrast to the'iirst loop which'foperated Y as an automaticV frequency control loop. VBy'properselec'- tion of the loopconstants Vtheloutput ofthelow pass'iilter;

canbeniadeidenticai to the outputof the lowrpassu'filtrery v fflZ'in th/eoverlap'region `of operation-where vbroth-modes Y rof operation are functioning. Theoyerlap rangeofoperVV -ationde`pends onthesensitivityof the'signallevelsampler Y Y andmicrowavediscriminator as; well as thefthresholds'let ting of threshold! circuit '221 c X/'henftheV thresholdl circuit'` 1 Y lis triggered by theoutputlfrom ,signal-level sa'rfnjpler'jl'ly andgthe diode microwave switch 'I is-open, the output of Vmatic phase control loop.vr Since .thegaurtornatic phase conf biasV Vcontrol, circuit i. j 'i in the altitude range up to greater than 50 feet, the altitude measurementis achieved with improved accuracy. As in the automatic frequency control mode, the modulation frequency is inversely proportional to altitude. The overlap range provides for continuity of operation between the two modes.

In order to ground monitor the altitude of an aircraft equipped with the radio altimeter of this invention, see FIG. 3, a series of receiving antennas 35, 36, 37 and 38, each equipped with the microwave discriminator heretofore described, are spaced along the approach path of the aircraft. The video signal derived from each antenna may be transmitted to a control location where the frequency can be converted to an altitude reading.

While I have described above the principles of my invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A distance measuring system comprising a transmitter for transmitting frequency modulated waves over a transmission path of unknown distance, a source of modulation signals, means coupling said modulation signals to modulate the output of said transmitter, a receiver adapted to receive the direct and the echo of said transmitted signals, transmission line means between said transmitter and said receiver for coupling a signal from said transmitter to said receiver which is approximately 180 out of phase with said direct signal, said receiver including means to detect the modulation signal in said received echo signal, a phase comparator coupled to receive the output from said detector means, means coupling a reference signal from said source to said comparator to thereby allow said comparator to compare the phase of said detected signal and the modulation signal coupled to said receiver, means responsive to said phase comparison to develop a control signal, means coupling said control signal to said modulation signal source to vary the output of said source to maintain a constant phase difference between said detected signal and said modulation signal.

2. A system according to claim 1 wherein said means to detect the modulation signal in said received echo signal includes a microwave discriminator having a hybrid ring coupler including an input section, an opposed terminating section and a pair of ports at right angles to said input and terminating sections, a transmission line crystal detector coupled to one of said ports and a transmission line coupled between the other port and said crystal detector.

3. A system according to claim 1 which further includes an automatic frequency control loop, said loop including means for mixing said direct and said echo signals coupled to said receiver to thereby produce a difference frequency, means for converting said difference frequency to a bias control signal, and means coupling said control signal to said modulation signal source to maintain a constant frequency difference between said detected signal and said modulation signal.

4. A system according to claim 3 which further includes means responsive to the amplitude of said received echo signal to disable said mixing means.

5. A system according to claim 3 and further comprising means for operating said phase comparison only when said received echo signal exceeds a predetermined level.

6. A distance measuring system comprising a transmitter for transmitting frequency modulated waves over a transmission path of unknown distance,

a source of modulation signals,

the output of said transmitter,

a receiver adapted to receive the direct and the echo l signals of said transmitted signal transmission line means between said transmitter and said reeciver for coupling a signal from said transmitter to said receiver which is approximately out of phase with said direct signal,

said receiver including means for separating said echo signal from said direct signal,

said receiver further including means to detect the modulation signal in said received echo signal,

a phase comparator coupled to receive the output from said detector means,

means coupling a reference signal from said source to said comparator to thereby allow said comparator to compare the phase of said detected signal and the modulation signal coupled to said receiver,

means responsive to said phase comparison to ldevelop a control signal,

and means coupling said control signal to said modulation signal source to vary the output of said source to maintain a constant phase difference between said detected signal and said modulation signal.

7. A distance measuring system comprising Y a transmitter for transmitting frequency modulated waves over a transmission path of unknown distance,

a source of modulation signals,

means coupling said modulation signals to modulate said receiver including means for receiving said 180 out of phase signal at one input terminal and said direct echo signals at another input terminal to thereby produce only said echo signal at an output terminal thereof,

said receiver further including means to detect the' modulation signal in said received echo signal, a phase comparator coupled to receive the output from said detector means, means coupling a reference signal from said source to said comparator to thereby allow said comparator to compare the phase of said detected signal and the modulation signal coupled to said receiver, means responsive to said phase comparison to develop a control signal, and means coupling said control signal to said modulation signal source to vary the output of said source to maintain a constant phase difference between said detected signal and said modulation signal. 8. A system according to claim 7 wherein said means for receiving said 180 out of phase signal comprises a hybrid ring.

References Cited in the file of this patent UNITED STATES PATENTS 2,498,933 Wallace Feb. 28,1950 2,520,553 Lawson Aug. 29, 1950 2,537,593 Landon Jan. 9, 1951 2,736,891 Munster Feb. 28, 1956 2,825,056 Rust Feb. 25, 1958 3,026,515 Rey Mar. 20, 1962 3,065,465 Wimberly Nov. 20, 1962 FOREIGN PATENTS 1,026,343 France Apr. 27, 1953 

6. A DISTANCE MEASURING SYSTEM COMPRISING A TRANSMITTER FOR TRANSMITTING FREQUENCY MODULATED WEAVES OVER A TRANSMISSION PATH OF UNKOWN DISTANCE, A SOURCE OF MODULATION SIGNALS, MEANS COUPLING SAID MODULATION SIGNALS TO MODULATE THE OUTPUT OF SAID TRANSMITTER, A RECEIVER ADAPTED TO RECEIVE THE DIRECT AND THE ECHO SIGNALS OF SAID TRANSMITTED SIGNAL TRANSMISSION LINE MEANS BETWEEN SAID TRANSMITTER AND SAID RECEIVER FOR COUPLING A SIGNAL FROM SAID TRANSMITTER TO SAID RECEIVER WHICH IS APPROXIMATELY 180* OUT OF PHASE WITH SAID DIRECT SIGNAL, SAID RECEIVER INCLUDING MEANS FOR SEPARATING SAID ECHO SIGNAL FROM SAID DIRECT SIGNAL, SAID RECEIVER FURTHER INCLUDING MEANS TO DETECT THE MODULATION SIGNAL IN SAID RECEIVER ECHO SIGNAL, A PHASE COMPARATOR COUPLED TO RECEIVE THE OUTPUT FROM SAID DETECTOR MEANS, MEANS COUPLING A REFERENCE SIGNAL FROM SAID SOURCE TO SAID COMPARATOR TO THEREBY ALLOW SAID COMPARATOR TO COMPARE THE PHASE OF SAID DETECTED SIGNAL AND THE MODULATION SIGNAL COUPLED TO SAID RECEIVER, 